S: (n) hairline (the natural margin formed by hair on the head)
S: (n) frontier (an international boundary or the area (often fortified) immediately inside the boundary)
S: (n) heliopause (the boundary marking the edge of the sun's influence; the boundary (roughly 100 AU from the sun) between the interplanetary medium and the interstellar medium; where the solar wind from the sun and the radiation from other stars meet)
S: (n) surface (the extended two-dimensional outer boundary of a three-dimensional object) "they skimmed over the surface of the water"; "a brush small enough to clean every dental surface"; "the sun has no distinct surface"
S: (n) Rubicon (the boundary in ancient times between Italy and Gaul; Caesar's crossing it with his army in 49 BC was an act of war)
S: (n) Mohorovicic discontinuity, Moho (the boundary between the Earth's crust and the underlying mantle) "the Mohorovicic discontinuity averages 5 miles down under oceans and 20 miles down under continents"
W: (v) edge [Related to: edge] (lie adjacent to another or share a boundary) "Canada adjoins the U.S."; "England marches with Scotland"
W: (v) bound [Related to: bound] (place limits on (extent or amount or access)) "restrict the use of this parking lot"; "limit the time you can spend with your friends"
S: (n) limit, bound, boundary (the greatest possible degree of something) "what he did was beyond the bounds of acceptable behavior"; "to the limit of his ability"